The following account of the prior art relates to one of the areas of application of the present invention, hearing aids.
In hearing aids (HA) with feedback cancellation, an adaptive filter can be used to estimate the part of the microphone signal that is due to feedback from the receiver (the signal path from the receiver to the microphone is typically termed the acoustic feedback path). The estimated signal is subtracted from the microphone input signal and the feedback is cancelled, if the adaptive filter has the same characteristics as the acoustic feedback path. There are several methods to update the adaptive filter. One commonly used method is to use the output signal as reference signal and the residual signal after cancellation as the error signal, and use these signals together with an update method of the filter coefficients that minimizes the energy of the error signal, e.g. a least means squared (LMS) algorithm, cf. FIG. 1a. This arrangement is termed ‘the direct method of closed loop identification’. A benefit of the direct method is that a probe noise is not necessary and that the level of the reference signal will be higher than if a probe noise is used. The drawback is that the estimate of the acoustic feedback path (provided by the adaptive filter) will be biased, if the input signal to the system is not white (i.e. if there is autocorrelation) or if improper whitening is used. This means that the anti feedback system may introduce artefacts when there is autocorrelation (e.g. tones) in the input.
The term ‘white’ in connection with acoustical or electrical signals is taken to mean that the signal has a substantially flat power spectrum in the frequency range of consideration.
Whitening can be used to avoid these artefacts. This is done by filtering both reference signal and error signal with a filter that makes the input signal without feedback component white. This filter should change with the spectrum of the input signal. Therefore it should be adaptive. Adaptive whitening is described by Spriet et al. in the paper “Adaptive feedback cancellation in hearing aids with linear prediction of the desired signal”. In this paper, the feedback cancellation is based on signals of the hearing aid, which does not enable the distinguishing of desired external tones and oscillations due to feedback.